New developments in telecommunication technology will require the bulk of telecommunication packaging to gradually shift from central office design to remote equipment design. This means that electronic packaging designers can no longer depend on a controlled atmospheric environment to ensure reliability of complex electronic systems. A very significant proportion of environment related reliability problems encountered by electronic packaging is either directly or indirectly related to atmospheric humidity. High humidity can give rise to the following effects: (i) degradation of thermal and electrical insulating materials, (ii) electrical shorts caused by condensation, (iii) oxidation and/or galvanic corrosion of metals, (iv) accelerated chemical reactions, (v) chemical or electrochemical breakdown of organic surface coatings, (vi) deterioration of electrical components, and others. It is therefore very important to provide design solutions for telecommunication packaging that provide for proper humidity management. As well, other environmental contaminants such as carbon dioxide may lead to shorter lifespans for electronic components.
One of the two basic approaches to humidity management of remote telecommunication equipment involves absolute humidity control (the other being relative humidity control). The idea behind relative humidity control is to maintain critical surfaces at a temperature higher than ambient by a constant .DELTA. T (temperature), while at the same time ensuring fast equilibration of internal and external absolute humidities.
Absolute humidity control is generally used in the presence of backpanels or other unheated boards with electrical bias, with inadequate energy dissipation, with variable telecommunication traffic and when no servicing is expected. Absolute humidity control is characterized by (i) using a non-permeable (usually metal) enclosure, (ii) allowing absolutely minimal amount of plastic or rubber for cabling and gasketing, (iii) allowing no openings, and (iv) using a desiccant to absorb any water or water vapour which enters the enclosure.
The construction of perfect hermetically sealed telecommunication packaging is seldom possible since the various seals in such packaging usually exhibit small leakages to allow moisture from the ambient atmosphere to enter the enclosure. The imperfections of such packaging are further heightened in so-called semi-sealed enclosures (used only in connection with relative humidity control) that have deliberate openings for reception of connectors, pass-through cables, etc.
Remote telecommunications equipment is often characterized by modules (both large and small) that are mated together to create electrical, electronic or optical connections. The reason for minimizing the size of such small modules to the point where they can no longer contain enough absorber for an extended product life is to increase dependability during shipping, installation and maintenance. The prior art reveals that it is possible to construct large sealed enclosures with a sufficient supply of desiccant to protect the electronic equipment contained therein for its lifetime. For example, in Shores U.S. Pat. No. 5,244,707 there is described a method of maintaining a dry atmosphere in a container type sealed electronic package by incorporating a coating or adhesive of desiccant properties.
However, it has heretofore been difficult to ensure the life of electronic equipment contained within relatively small enclosures that are to be mated with other relatively larger enclosures. Once mated with a relatively larger enclosure, the diminished humidity control characteristics of such relatively small enclosures risk the viability of the electronic equipment enclosed within both enclosures.
A number of desiccant caps have been designed to provide a low humidity atmosphere within an associated container. For example, in Santoro U.S. Pat. No. 4,350,508, the desiccant cap comprises an enclosure having an internal cavity divided into two portions, the first of which is designed to accept and hermetically seal a container, and the second designed to accept and hold a mixture of desiccant and a humidity sensitive colour indicator. Though other desiccant caps may be designed for use with remote telecommunication equipment, the desiccant caps disclosed in Santoro U.S. Pat. No. 4,350,508 have no application for the protection of remote telecommunication equipment.
In Dalgleish U.S. Pat. No. 5,548,643 (related to relative humidity control) there is described an outdoor wireless base station having a housing and a circuit pack extending upwards in the housing with electronic components extending into a forward air flow passage. A humidity control device is used to minimize moisture problems within the housing by providing a moisture balance between the inside and outside of the housing. While useful for controlling relative humidity, this invention would not be applicable in an environment where absolute humidity is sought to be controlled, e.g., in small, low powered modules.